Reversible perovskite-fluorite phase transition in alumina-supported CeFeOx films

Abstract

Thin films with a stoichiometry of CeFeO_x were conformally deposited on high-surface-area γ-Al₂O₃ by Atomic Layer Deposition (ALD). X-ray diffraction (XRD) patterns, High-Resolution Transmission Electron Microscopy (HRTEM) images, Raman spectra, and Mössbauer spectra demonstrated that 2 nm-thick films exhibited a perovskite structure after reduction at 1073 K but converted to a fluorite phase upon oxidation at 1073 K. The transition between the fluorite and perovskite structures was reversible for at least five oxidation and reduction cycles. Coulometric titration at 1073 K showed that reduction of the fluorite phase occurred in two steps, one at a P(O₂) of 10⁻¹⁵ atm and a second at a P(O₂) of 10⁻⁸ atm. X-ray Photoelectron Spectra (XPS) demonstrated that Ce has +3 valence in the perovskite phase and +4 valence in the fluorite phase, while Fe is mixed +2 and +3 valence in the reduced perovskite phase and +3 valence in the fluorite phase. The CeFeO_x thin films were found to retain high surface area and remain conformal to the γ-Al₂O₃ support upon redox cycling suggesting that they may be useful in applications ranging from catalysis to spintronics.